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FT-NIR spectrometer | SIMTRUM Photonics Store

Products / Light Analysis /Spectrometers /FT Infrared Spectrometer(900-16000nm) /FT-NIR spectrometer(900-2500nm) /
FT-NIR spectrometer(900-2500nm)
Overview Specifications Working Principle Software Application Q&A Related Products Order Now！

SIMTRUM'S FT-NIR spectrometer is a high-performance system combining high resolution, broad wavelength range and excellent sensitivity. The FT-NIR functioning principle permits to measure over a much broader range than grating spectrometers that are limited to 2100nm.

Thanks to its permanently aligned interferometer and solid-state reference laser, the FT-NIR Rocket offers excellent stability in both intensity and wavelength scales. The FT-NIR Rocket is compatible with light sources and sampling accessories (like fibers, cuvette folder, ...) typically used with array-detector based NIR spectrometers.

Its outstanding stability in both wavelength and intensity scales makes it an ideal tool for highly reproducible chemometric analysis.In comparison to array-photodiode NIR spectrometers, the single photodiode operation of the FT-NIR ensures no defect pixels, no gain variation among pixels and no dark-current drifts. Also, the device does not suffer from stray-light as grating spectrometers.

With four available spectral ranges and adjustable spectral resolution down to 2cm^-1, the FT-NIR spectrometer is a highly flexible instrument that can be tailored to your application. Designed for convenience and ease-of-use, our FT-NIR spectrometer is readily operational with our software using a standard USB 2.0 connection.

Features

  • Wide wavelength range(0.9-2.5um)
  • High resolution
  • Excellent stability in intensity & wavelength scale
  • High sensitivity
  • Compact and rugger
  • Long lifetime
  • Low power consumption

Application

  • Transmission, diffuse reflectance measurement
  • Light source measurement
  • Material identification and quantification

 

 

 
 

Principle

Permanentely aligned interferometer

The heart of the FT-Rocket is dual corner-cube (retro-reflector) interferometer. The two corner-cubes are fixed to a common swinging arm, which rotates to create an optical path difference (with respect to the beam splitter) in the two arms of the interferometer.This type of design is called a permanently aligned interferometer. This particular arrangement of the interfeormeter is known to be the most robust against vibrations and temperature drifts. It never has to be realigned. The swinging arm of the interferometer rotates on wear-free flexure system, making this mechanical system extremely robust and durable.

 

Solid-state reference laser

For measuring the movement of the mirrors, a solid-state reference laser is coupled into the interferometer. Compared to classic HeNe lasers, the solid-state lasers that we use are more compact and have a much longer life-time. They have a very low temperature-induced wavelegnth drift and, when kept at constant temperature with a Peltier element, their wavelength can be stabilized to a few PPM, thus providing a very accurate and reproducible wavelength scale. This is crucial for ensuring a day-to-day and unit-to-unit consistency.


General Parameters
Model LA-AR01-FTNIR-025
  Spectral Range [cm-1] 11000-4000
  Spectral Range [μm] 0.9-2.5
  Detector Type Extended type InGaAs 2-stage TE-cooled
  Detector Peak D*[cm Hz1/2 W-1 ] ＞2×10^11
  Signal-to-noise ratio ＞100000:1
  Recommended fiber Low-OH silica Multi-mode fiber with 200µm or 600µm core
  Internal reference laser Temperature controlled solid-state @795nm
  Resolution(unapodized) [cm-1] 2, 4, 8 (user selectable)
  Wavenumber repeatability ＜20 PPM
  Scan frequency ＞4 Hz @ 4cm-1
  Power requirement 12V / 10W max

 

 

Other Parameters

  Fibered interface Fiber core up to ∅ 0.6mm, NA=0.25, SMA 905 connector
  Interferometer type Permanently aligned with dual retro-reflector
  A/D Converter 24 bit
  Amplifier 4 gain levels low noise trans-impedance amplifier
  Operating temperature / humidity 5℃-40℃ / non condensing
  Communication Interface USB 2.0
  Software Interface Windows 7/10/11 API for controlling the instrument via our DLL
  Dimensions 180mm×160mm×80mm
  Weight 1800g

 


FT-IR spectroscopy: high resolution measurements

Introduction

Fourier Transform infrared (FT-IR) spectrometers have proven to be efficient and reliable tools for a large variety of applications targeting the near infrared (NIR) and mid-infrared (MIR) regions of the electromagnetic spectrum. One of the most critical specification of FT-IR spectrometers is their spectral resolution , as this defines the scale of the features to be distinguished e.g. in the absorbance spectrum of a gas. It is thus quite natural to wish for the highest achievable resolution when purchasing an FT-IR spectrometer. Due to their operational mode, achieving high resolution measurements using FT-IR spectrometers is however not entirely straightforward. This technical note explains the main limits of high resolution FT-IR measurements.

 

Fig .1 Simplified FT-IR setup

 

A simplified FT-IR is depicted in Fig.1. It consists of a beamsplitter, a fixed mirror and a movable mirror. Light from a purely monochromatic light source is split in two beams, assumedly in equal parts (50% beamsplitter). Each beam bounces back on either mirror (fixed or movable) before being recombined and focused onto the detector. When the movable mirror is at the same distance from the beamsplitter as the fixed mirror, both beams travel the same distance and recombine in-phase, yielding constructive interference. By displacing the moving mirror by a distance ℓ, one introduces an optical path difference (OPD) between the two beams Δ given by:Δ=2ℓ.

Upon recombination at the detector, the optical intensity as a function of OPD is given by: I(0)=0.5I0[1+cos(2πν0∆)]=IDC+IAC(∆).

Where I0 is the source intensity at wavenumber ν0 (the wavenumber is the reciprocal of the wavelength). The AC part of the interference record is labelled IAC(Δ) and is called the interferogram. For a purely monochromatic source (as considered here), the interferogram is a pure cosine function.

 

Fig 2. Interferogram of a purely monochromatic light source

 

Here the wavenumber (or equivalently the wavelength) of the source as well as its intensity can be retrieved from a direct observation of the interferogram (amplitude and period of the cosine function). For a broadband source, the interferogram IAC(Δ) and spectrum I0(ν) of the source are related via a Fourier transform operation:

I0(v)=∫∆maxIAC(∆)cos(2πν0∆)d∆

Obviously, the OPD cannot be made arbitrarily large and has to reach a value Δmax that is defined by technological design. Given the nature of the relationship between the interferogram and the spectrum (Fourier transform), it turns out that Δmax also defines the achievable spectral resolution. To a first approximation, the spectral resolution Δν of an FT-IR is given by:

∆ν=(∆max)-1

and is often expressed in cm-1. So why not simply increase the maximum OPD to enhance the spatial resolution of an instrument ? While this is true, special care has to be considered when operating at high resolution. As explained hereafter, the mirror maximum displacement is limited by the divergence of the system and the dimensions of the detector.

 

 

HR measurement: effect of beam divergence

We consider the exact same setup as in Fig.1. The beam divergence is accounted for by observing the behavior of the so called "extreme ray", which makes an angle α with respect to the "central ray" discussed previously.

 

 

Fig 3. FT-IR setup with a divergent source

 

 

The two extreme rays (reflected from either the fixed or the moving mirror) hit the lens with an angle α, unlike the central ray which hits the lens at normal incidence. They are thus focused on another point on the detector. Moreover, their OPD is shorter than for the central ray:

∆ext=2ℓcos(α)

The larger the angle α, the greater the difference with the central ray OPD Δcen=2ℓ. Consider now the case where the difference in OPD between the central ray and the extreme ray is equal to one half of the source wavelength, that is:

∆cen-∆ext=2ℓ[1-cos(α)]=λ/2

In this scenario, when the central rays are in phase, then the extreme rays are out of phase (and vice versa). Consequently, the intensity over the detector surface follows the profile shown in Fig. 4.

 

Fig 4. Intensity over the detector surface due to a highly diverging beam

 

 

Since the detector yields a single value that corresponds to the average intensity received on its surface, the signal detected in this case corresponds to the average optical intensity only, and all information regarding the interference signal vanishes. Practically speaking, the interferogram will start losing contrast as the moving mirror is scanned as shown in Fig. 5.

 

 

Fig 5. Loss in interferogram contrast due to a diverging beam

 

 

This effect is naturally existing in all interferometers based instruments (such as FT-IR) and cannot be avoided. It can however be properly managed by appropriately trading-off the parameters involved in equation (6), namely :

  • Δν: the effect is more pronounced for high resolution measurements due to the larger OPD required.
  • α: the effect is more pronounced for a highly diverging beam.
  • λ: the effect is more pronounced at short wavelengths (large wavenumbers).

 

For most applications in solids and liquids, the size of the observable features is typically broader than 2cm-1, and high resolution (HR) measurements performed at 0.5cm-1 are usually not required. In addition, these would prove challenging due to the added contrast loss described in this document. HR measurements might still be a viable option for specific applications, such as e.g. laser characterization, where the highly collimated laser beam prevents the dramatic loss of the interferogram contrast.


We fully appreciate and value the multiple benefits that a dedicated, performant and reliable software can bring to your application. Automatic data collection, parameters changes, status diagnosis and many other essential tasks should be implemented as simply and as efficiently as possible in order to get the most out of your spectrometer. This philosophy led to the development of a multi-threading, cross-platform and versatile software application, the digital acquisition system or AoDAQ.

The AoDAQ simultaneously takes care of:

1. Handling communication with the FT-IR via USB

2. Processing raw signals to deliver a spectrum

3. Running a TCP Ethernet server

The AoDAQ can be installed on all sorts of computers, from desktop machines to embedded, low-power single board computers. Thanks to the hosting of a TCP server, the instrument data and parameters can be accessed locally and/or remotely. All communication with the instrument eventually reduces to a set of TCP/IP commands that allow to quickly acquire data, adjust parameters, monitor the instrument status etc. using the programming environment of your choice. 

 


Case1：Mineral identification

 

Experimental Setup：

  • LA-AR01-FTNIR-025: FTIR spectrometer with 2TEC 0.9-2.5µm InGaAs detector
  • LA-AR01-AC-sphere-50-Hal: an internally illuminated integration sphere (it has a light bulb inside) with a 10mm diameter measurement port

FTNIR 0.9-2.5 μm with the internally illuminated integrating sphere-50-Hal

 

Measurements Result:

After taking the usual dark and baseline measurements, the reflection spectrum of the sample rock is easily obtained within a few seconds, simply by placing the rock sample onto the measurement port of the integrating sphere. Note that the integrating sphere measurement port is closed by a sapphire window. This avoids that the inside of the integrating sphere is contaminated, and the window is not scratched easily.

 

Below are a few examples of spectra collected in 10 seconds with the above describes system, to give an ides on the signal to noise ratio and resolution.

Reflectivity of rocks

 

Case 2:Diffuse Reflectance Measurement

 

Experimental Setup：

  • LA-AR01-R7-VIS-600-150F：a Y-branched fiber bundle probe
  • LA-AR01-FTNIR-025: FTIR spectrometer with 2TEC 0.9-2.5µm InGaAs detector
  • LL-AR01-NIR: the source is a 20 W broadband lamp

The probe is maintained fixed by a dedicated mount which ensures stability and reproducibility.

 

 

Case 3：Transmission measurement

Transmission spectroscopy is a widespread analysis technique that consists in measuring the absorption produced by a given sample when a beam of infrared light goes through them. Transmission spectroscopy applies mostly to liquids and gases, yet pressed powders or thin-films might also be characterized using this methodology. Besides spectroscopic measurements, transmission measurements are also useful to evaluate the properties of optical components, such as windows and filters.

 

Experimental Setup：

NIR transmission spectroscopy setup.

  • LL-AR01-NIR：A fiber coupled light source based on a QTH bulb
  • LA-AR01-AC-CUV: can host cuvettes wild optical paths going up to 1cm. It also features a slot dedicated to additional filters (intensity or color) or other samples (thin-films) that can be conveniently mounted with or in place of the cuvette.
  • LA-AR01-FTNIR-025:FTIR spectrometer with 2TEC 0.9-2.5µm InGaAs detector

1.What resolution should I use for my application?

In FTIR, resolution is traded-off with two other experimental metrics that are acquisition time and signal-to-noise ratio (SNR). Increasing resolution, meaning reducing the resolution parameter number, will result in longer acquisition time, and poorer SNR. In general, it is recommended to work at the "worst" possible resolution, that is the limit resolution that allows to distinguish the features of the sample or substance that you are characterizing. Liquids and solids present broader features than gases or gas mixtures and can generally be probed with standard resolution instruments (down to 2cm-1). Gas analysis or light sources characterization (typically lasers) usually benefit from a sharper resolution of 0.5cm-1.

 

2.How is the equivalent wavelength resolution calculated?

Due to its working principle, FT-IR provides uniformly sampled spectra in the form of wavenumber (ν) within a given spectral range, with the unit of cm-1. Wavenumber is simply defined as the reciprocal of wavelength (λ). The resolution of a FT-IR is a constant wavenumber (Δν), but varies with the wavelength (Δ λ) due to the inverse relationship between these two units. The conversion between wavenumbers resolution and wavelengths resolution is Δλ=λ2 · Δν, as shown in the following figure:


 

3.What is the acquisition rate of this spectrometer?

The acquisition rate varies with the resolution. At the standard resolution of (4cm-1), the scanning rate is ~5Hz, and at the high resolution of (0.5cm-1), the scanning rate is ~1Hz, that is, the scanning rate is inversely proportional to the resolution.

 

 

4.What are the differences between DLADTGS detectors and MCT detectors?

DLADTGS is based on the pyroelectric effect. When exposed to infrared radiation, its temperature will change, causing the polarization inside the crystal to change. MCT is a bandgap type photoconductive detector. DLADTGS has a wider spectral response range (up to 18-20 μm), and MCT detectors utilize thermoelectric cooling to achieve higher sensitivity, which can suppress dark current and improve their signal-to-noise ratio.

 

 

5.How much better is the performance of LN4 cooled detectors than that of MCT cooled detectors?

The SNR of the LN4 cooling system is approximately ten times that of the MCT cooling system.

 

 

6.Why is the performance of the Fourier transform spectrometer superior to that of the dispersive grating spectrometer?

The performance of a grating spectrometer mainly depends on the slit and detector. Due to the different dark noise of each pixel point, additional noise will be introduced during measurement. FTIR spectrometers are generally superior to dispersive grating spectrometers due to their higher light throughput, better signal-to-noise ratio, higher spectral resolution and wider wavelength range. These advantages make FTIR spectrometers the preferred choice in many applications, especially in places where high sensitivity and accuracy are required.

 

 

7.Can an FTIR spectrometer be used to measure pulsed laser?

Sure, but the power of the laser must be limited to avoid irreversible damage to the detector. The average power shall not exceed 25mW, and the peak power of pulses shorter than 1µs shall not exceed 100W. It is recommended to use a set of fixed or variable attenuators to adjust the optical power to avoid detector saturation. Secondly, the repetition frequency of the laser must exceed 25khz to avoid numerical artifacts (aliasing) in the measured spectrum.

 

 

8.Can the concentration of the substance being measured be obtained directly?

No. A spectrometer can only provide the measured spectrum. Users must obtain the concentration of substances in the sample through specific algorithms and calibration data.

 

 

9.Can this spectrometer be used for mineral identification?

The FT-NIR spectrometer is highly suitable for mineral identification. FT-NIR is capable of generating high-quality and high-resolution (better than 1nm) reflection spectra within the spectral range of 900-2550nm (SWIR) within a few seconds.


Related products

Model Wavelength Range Resolution Detector  
  FT-NIR spectrometer 900-2500 nm 2/4/8 cm-1(user selectable) Extended type InGaAs PIN photodiode, 2TE cooled
  FT-MIR spectrometer 200-16000 nm 0.5/2/4/8cm-1(user selectable) MCT (4-TE cooled)
MCT (LN2 cooled)
DLATGS
  Fiber coupled FT-IR spectrometer 200-16000 nm 2/4/ 8cm-1 (user selectable) InGaAs (2-TE cooled)
MCT (4-TE cooled)
MCT (LN2 cooled)
  VIS-NIR-FIB spectrometer 350-2500 nm <1.5 nm silicon array detector (3648 pixels) 16-bit ADC.extended range InGaAs photodiode, 2TE cooled, 24-bit ADC.
  VIS-NIR-DR spectrometer 360-2500 nm 5 nm Extended range InGaAs detector

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LA-AR01-FTNIR-025-2TE
Spectral Range：0.9-2.5μm,Detector Peak D*：＞2×10^11cm Hz^1/2 W^-1,Signal-to-noise ratio：＞100000:1,Resolution：2/4/8 cm-1(user selectable) 		4-6week $24960.00

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